Aptamers, which are nucleic acid ligands capable of binding to molecular targets, have recently attracted increased attention for their potential application in many areas of biology and biotechnology. They may be used as sensors, therapeutic tools, to regulate cellular processes, as well as to guide drugs to their specific cellular target(s). Contrary to the actual genetic material, their specificity and characteristics are not directly determined by their primary sequence, but instead by their secondary and/or tertiary structure. Aptamers have been recently investigated as immobilized capture elements in a microarray format. Others have recently selected aptamers against whole cells and complex biological mixtures. Aptamers are typically characterized by binding to their target molecules via non-Watson-Crick (i.e. non-hybridization) mechanisms, such as by intermolecular forces resulting from the secondary or tertiary structure of the aptamer. This is especially true of non-nucleic acid target molecules where Watson-Crick mechanisms typically do not apply.
Aptamers are commonly identified by an in vitro method of selection sometimes referred to as Systematic Evolution of Ligands by EXponential enrichment or “SELEX”. SELEX typically begins with a very large pool of randomized polynucleotides which is generally narrowed to one aptamer ligand per molecular target. Once multiple rounds (typically 10-15) of SELEX are completed, the nucleic acid sequences are identified by conventional cloning and sequencing. Aptamers have most famously been developed as ligands to important proteins, rivaling antibodies in both affinity and specificity, and the first aptamer-based therapeutics are now emerging. More recently, however, aptamers have been also developed to bind small organic molecules and cellular toxins, viruses, and even targets as small as heavy metal ions.
Citrulline is an a-amino acid. It is a key intermediate in the urea cycle, the pathway by which mammals excrete ammonia. In the human body, citrulline is produced as a byproduct of the enzymatic production of nitric oxide from the amino acid arginine, catalyzed by nitric oxide synthase. Several proteins contain citrulline as a result of a posttranslational modification. These citrulline residues are generated by a family of enzymes called peptidylarginine deiminases (PADs), which convert arginine into citrulline in a process called citrullination or deimination. Proteins that normally contain citrulline residues include myelin basic protein (MBP), filaggrin, and several histone proteins, whereas other proteins, such as fibrin and vimentin are susceptible to citrullination during cell death and tissue inflammation. Circulating citrulline concentration is a biomarker of intestinal functionality.
Crystallin is a water-soluble structural protein found in the lens and the cornea of the eye accounting for the transparency of the structure. Alpha-crystallin occurs as large aggregates, comprising two types of related subunits (A and B) that are highly similar to the small (15-30 kDa) heat shock proteins (HSPs), particularly in their C-terminal halves. The relationship between these families is one of classic gene duplication and divergence, from the small HSP family, allowing adaptation to novel functions. Divergence probably occurred prior to evolution of the eye lens, alpha-crystallin being found in small amounts in tissues outside the lens. Alpha-crystallin has chaperone-like properties including the ability to prevent the precipitation of denatured proteins and to increase cellular tolerance to stress. It has been suggested that these functions are important for the maintenance of lens transparency and the prevention of cataracts. This is supported by the observation that alpha-crystallin mutations show an association with cataract formation. Alpha crystallin B-chain (CRYAB) is one of the subunits that forms alpha-crystallin.